Binders prepared by emulsion copolymerization of vinyl acetate and ethylene in the presence of a crosslinking agent selected from among N-methylol and N-alkoxy compounds, as well as products employing such binders are known. French Pat. No. 1,564,100 teaches efforts directed to increasing the solvent resistance and improving the enhanced mechanical stability of a film prepared from a vinyl acetate/ethylene copolymer emulsion. The efforts comprise using an N-methylol compound (e.g. N-methylolacrylamide or N-methylolmethacrylamide) or an N-alkoxy compound (e.g. a methyl or butyl ether of said N-methylol compound) as a cross-linking agent in an amount ranging from 0.5 to 10 wt % of vinyl acetate, and subjecting the copolymer to cross-linking at temperatures between room temperature and 180.degree. C., preferably in the presence of a curbing agent. U.S. Pat. Nos. 3,345,318 and 3,380,851 describe the emulsion polymerization of a vinyl acetate/ethylene/N-methylolacrylamide copolymer and state that the resulting emulsion is useful as a binder in nonwoven fabrics and paper products having high resistance to water and solvents.
As shown in Japanese Patent Application (OPI) No. 55709/1977 (the term "OPI" as used herein means an unexamined published Japanese patent application), nonwoven fabrics and impregnated papers employing the aforementioned emulsion as a binder exhibit high resistance to water and solvents and yet retain the inherent properties of the vinyl acetate/ethylene copolymer, such as high resistance to light and heat and good embossability. Because of these advantages, the nonwoven fabrics and impregnated papers employing the emulsion as a binder are replacing increasingly those products which use such synthetic rubber latices as conventional styrene/butadiene copolymers, acrylonitrile/butadiene copolymers or methyl acrylate/butadiene copolymers as binders and which have low resistance to heat and light, as well those which employ acrylic copolymer emulsions as binders and which exhibit only poor heat resistance and embossability.
Nonwoven fabrics have many advantages over woven fabrics, such as low manufacturing cost, fray resistance and the capability of being provided with even higher degrees of water absorbency, porosity and resiliency. Because of these advantages, nonwoven fabrics find much utility in many end uses such as towels, diapers, wiping cloths, sheets, sanitary napkins, masks, gowns and covering spreads for use in hospitals, disposable wet towels, tableclothes, aprons, interlinings, and filters for civil-engineering purposes.
While nonwoven fabrics may be fabricated by a variety of techniques such as adhesive bonding, needle punching, spunlacing, stitch bonding and spunbonding, the present invention is primarily directed to the adhesive bonding type. This type of nonwoven fabric is manufactured by first preparing a web of loosely bound filaments and then giving them integrity with the aid of a binder which is deposited on the web by impregnation, printing, spraying or any other appropriate method. The so treated web is subsequently dried. The filaments may be made of such fibers as natural cellulosics, polyesters, polyamides, polyacrylics, rayons, wool, jute, and mixtures thereof. The filaments are loosely bound to a web form by known techniques such as carding, garnetting, air-laying, and papermaking processes.
The binder which is deposited on the web by impregnation, printing, spraying or any other suitable method is employed with a medium which is made of either water alone or the combination of water and air. The second type of medium is used when a conventional binder composition in emulsion is deposited after being foamed (this deposition technique is hereinafter referred to as the foam deposition process). In order to allow a given solids content of binder to be deposited over a large area, the binder must be diluted with water if the medium is solely made of water. On the other hand, there is no need to dilute the binder with water if the medium is made of both water and air because the volume of the binder can be increased by foaming. Therefore, the foam deposition process has the advantage of providing a higher solids content of binder. Since less water needs to be evaporated in the drying step, a significant cost reduction is realized with respect to the energy for drying. The low water evaporation offers another advantage in that minimum "migration" (i.e., the solid matter in emulsion accompanies the evaporating water and is carried to the surface) which affords increased strength for the adherend. Because of these advantages, the foam deposition process is gaining increasing attention of the textile industry.
As described above, a certain improvement in resistance to water and solvents can be attained by copolymerizing vinyl acetate with ethylene in the presence of a cross-linking agent selected from among N-methylol compounds such as N-methylolacrylamide and N-methylolmethacrylamide, and N-alkoxy compounds thereof. However, in many end uses even better resistance to water and solvents is desired. Finished nonwoven fabrics and impregnated papers manufactured today will not emit the malodor of formaldehyde but they contain a detectable amount of toxic formaldehyde and may present a health hazard if they are used in applications where they make direct contact with the skin.
It is therefore desired to further reduce the formaldehyde content in nonwoven fabrics and impregnated papers (the residual free formaldehyde is hereinafter abbreviated as FF) while satisfying the requirement for higher resistance to water and solvents. In order to increase the resistance to water and solvents, the density of crosslinking in the binder must be first increased and this requires that the content of a monomer such as N-methylol or N-alkoxy compound which is involved in crosslinking should be increased. However, as the contents of these monomers increase, the FF level increases inevitably, and it is impossible to meet the two requirements (i.e., higher resistance to water and solvents, and lower FF) simultaneously by conventional techniques. These heretofore incompatible demands can be met by the binder composition of the present invention. In addition, the more foamed the binder composition is in the foam deposition process, the larger the volume of the binder and the smaller the solids content of binder that is necessary for attaining uniform deposition of the binder over a given area. In other words, the loading of a solid matter in the binder can be increased by allowing it to be foamed to a greater extent. It has therefore been desired to realize a maximum degree of foaming in addition to the provision of higher resistance to water and solvents and lower FF. This objective has been unattainable by any prior art techniques but can be achieved by the binder of the present invention if it is foamed to a great extent.